A gearbox malfunction was the last thing on our customer's mind when he got into his E46 M3 and discovered it wouldn't start. There are many more frequent explanations for a no start – a dead battery, worn out starter, malfunctioning brake pedal switch, and so on – and after ruling out the obvious candidates and failing to locate the issue, he had the car towed to our service.
What is the BMW SMG Transmission's mechanism?
For those unfamiliar with BMW's SMG transmission, the best way to explain it is that it is neither a traditional "manual" nor a "automatic" transmission, but rather a hybrid of the two. There is no clutch pedal, but you can change gear using a "stick" (or paddles positioned right behind the steering wheel) much like you would in a regular standard. Instead, a computer controls the clutch, which is then operated by hydraulics.
Symptoms and Diagnosis: How to tell if an SMG pump has failed and requires replacement.
We connected the M3 to our computer diagnostics scan tool and discovered a DTC, or "diagnostic trouble code," indicating a problem with the hydraulic pressure, which is related to failing integral components on the SMG hydraulic pump assembly.
The SMG pump looks and works like an ABS pump, except it controls hydraulic pressures in the transmission rather than the brake system. We discovered that the huge, 40 amp fuse for the SMG pump (shown below) had blown with that information to send us in the right path. This is a very common and strong indication that the pump assembly has failed. We replaced the fuse and started the car to verify our theory, and the new fuse blew practically instantly.
What causes the car to not start when the SMG pump fails?
When the pump fails and the aforementioned fuse blows, the on-board computers that monitor and regulate the pump lose control with it and cannot control gear shifts. The clutch is engaged and unusable by the automobile's computers even if the car is in neutral. It's the same reason why you can't start a modern manual gearbox automobile unless the on-board computer receives a signal from the clutch switch indicating that the pedal has been pressed all the way in, disengaging the clutch before turning the engine over. Similarly, you can't start because of the same principle.
BMW SMG Pump Assembly Replacement
We replaced the SMG pump with our unit after determining that it was bad, cleared the codes from the on-board computer, and performed some QA/QC with a test drive to ensure that the problem was fixed and the vehicle was operating properly.